The preferential retention of ^VIZn over ^IVZn on birnessite during dissolution/desorption

Zn is a common heavy metal in soils and sediments. In this study, the release behaviors of octahedral (VIZn) and tetrahedral (IVZn) Zn complexes on synthesized hexagonal birnessite were explored by solution chemistry method in combination with spectroscopic analysis. In acidic dissolution processes, the release of adsorbed Zn2+ from birnessite occurred into two stages: in the first stage, ~60% of Zn2+ was desorbed rapidly, with only 8% of Mn being released, and the ratio of VIZn/IVZn increased with time; in the second stage, the residual Zn2+ was mostly VIZn and released slowly at a nearly constant rate until complete dissolution of the matrix mineral was observed. During desorption of Zn2+ by Pb2+, the ratio of VIZn/IVZn on birnessite also increased, while the residual percentage of VIZn remained nearly constant. However, it is known that IVZn-triple corner-sharing (TCS) is more stable than VIZn-TCS, suggesting that part of the remaining IVZn-TCS on birnessite might transform to VIZn-TCS immediately when VIZn-TCS is replaced by H+ or Pb2+. Additionally, the possible distribution of Mn3+ and IVZn or the partial charge compensation by protons can lead to the preferential retention of VIZn on birnessite or the preferential re-adsorption of VIZn at the new edge sites. These results can provide new insights into the geochemical behavior of Zn2+ contaminant in soil and aquatic environments